[Thread] add private API for disabling border router

Bug: 374036806
Change-Id: Ib8ca8bbd2b3018c39c9250e1b3ce70b193273c1b
diff --git a/common/thread_flags.aconfig b/common/thread_flags.aconfig
index 60120bc..8cc2bb4 100644
--- a/common/thread_flags.aconfig
+++ b/common/thread_flags.aconfig
@@ -44,3 +44,12 @@
     description: "Controls whether the setConfiguration API of NAT64 feature is enabled"
     bug: "368456504"
 }
+
+flag {
+    name: "thread_mobile_enabled"
+    is_exported: true
+    is_fixed_read_only: true
+    namespace: "thread_network"
+    description: "Controls whether Thread support for mobile devices is enabled"
+    bug: "368867060"
+}